Fuel cells are devices which convert chemical energy directly into electrical energy by placing a pair of electrodes in mutual contact through an intervening electrolyte, feeding a fuel to one of the electrodes and an oxidant to the other electrode, and carrying out oxidation of the fuel electrochemically within the cell. There are several types of fuel cells, depending on the electrolyte used. Solid polymer fuel cells in which the electrolyte is a solid polymer electrolyte membrane have attracted considerable attention recently for their ability to achieve a high energy output.
As shown in FIG. 1, such solid polymer fuel cells are composed of a stack of from several tens to several hundreds of unit cells. Each unit cell has a pair of fuel cell separators 1, 1 with a plurality of ribs 1a on either side thereof. Between the separators 1 are disposed a solid polymer electrolyte membrane 2 and a pair of gas diffusing electrodes (a fuel electrode and an oxidant electrode) 3, 3.
Of the components making up this type of fuel cell, the fuel cell separator has the distinctive shape of a thin plate provided on one or both sides thereof with a plurality of flow channels 4 for the supply and removal of gases. The separator plays several important roles, one of which is to separate the fuel gas, oxidant gas, and cooling water flowing through the fuel cell to keep them from mingling. In addition, it carries away electrical energy generated by the cell, and dissipates to the exterior heat that has formed within the cell. Accordingly, a need has been strongly felt for fuel cell separators which, in addition to having gas barrier properties, electrical conductivity and corrosion resistance, also have sufficient mechanical strength to resist cracking and breaking of the separators from the tightening of bolts and nuts during fuel cell assembly, and which moreover are endowed with excellent vibration and impact resistance when the fuel cell is used as a mobile power supply such as in automobiles.
Known methods for manufacturing such fuel cell separators include processes in which the separator is fabricated by:    1) molding a thermosetting resin, heat-curing the molded resin, then machining;    2) machining dense carbon that has been impregnated with a thermosetting resin;    3) laminating and pressing together carbon fiber nonwoven fabrics impregnated with an electrically conductive coating;    4) working together a carbon powder and a phenolic resin, then molding the resulting blend under heat and pressure;    5) blending together a carbon powder, a phenolic resin and carbon fibers, then compression molding the blend.
However, when a heat-cured resin product or resin-impregnated dense carbon is machined, the costs of machining are high. Moreover, separators having a small thickness are subject to breakage during machining and fuel cell assembly. In prior-art processes that involve molding a conductive coating-impregnated carbon fiber nonwoven fabric, the nonwoven fabric interferes with the shaping of grooves.
In processes which involve molding a mixture of graphite powder and resin, increasing the proportion of conductive filler so as to enhance the electrical conductivity lowers the moldability and mechanical strength. On the other hand, increasing the proportion of binder resin to improve moldability and mechanical strength lowers the electrical conductivity. Methods for producing separators from a mixture of graphite powder, phenolic resin and carbon fibers do enhance the strength, but the resulting separators have a very high flexural modulus, making them subject to breakage when thin.